What is the width (in pixel dimensions) of a column in the 3-column layout (in the shortcode)? I’m creating images that I want to span the entire width of the column. I guess to prevent further forum posts, if it becomes necessary, could you tell me the width of the 2 and 4 column layout as well?

That is a bit hard to say. The shortcode columns are actually percentage based so they don’t have a defined value. You should be able to use a larger image than the column size and have it scale down though it *may* require adding a class to adjust the image depending on the exact set up.